(CNN) -- A moderate Senate Democrat said Sunday he could support a health-care bill that includes a provision for possibly bringing in a government-funded public health insurance option in the future.

Sen. Ben Nelson said a trigger mechanism would serve as a "fail-safe backstop."

The comment by Sen. Ben Nelson of Nebraska on CNN's "State of the Union" signaled a possible compromise on the most contentious issue in efforts to overhaul the nation's ailing health-care system.

The so-called trigger mechanism would create a public insurance option if a new health-care bill fails to meet certain goals or thresholds for expanded coverage and lower costs within a set time.

"If, somehow, the private market doesn't respond the way that it's supposed to, then it would trigger a public option or a government-run option," Nelson said, adding such a provision should be used "only as a fail-safe backstop to the process."

"And when I say trigger ... I don't mean a hair trigger," Nelson said. "I mean a true trigger -- one that would only apply if there isn't the kind of competition in the business that we believe there would be."

Support from moderate Democrats such as Nelson is considered crucial for getting health-care legislation that includes a public option through the Senate. Republicans fiercely oppose the concept, while liberal Democrats strongly favor it and more moderate Democrats express concerns about the cost and viability.

So far, three House committees and one Senate committee have passed Democratic proposals containing a public option, while members of another Senate committee are negotiating a bipartisan compromise that drops the public option.

Don't Miss

However, one of those Senate negotiators -- moderate Republican Olympia Snowe of Maine -- has been in discussions with the White House on the trigger proposal for a public option. The possible backing from Snowe signals a middle-ground for Nelson and other moderate Democrats on the issue.

On the House side, House Speaker Nancy Pelosi -- who has insisted on a public insurance option in any health-care bill -- last week stopped short of ruling out a triggering mechanism similar to what Nelson described. Asked by reporters about the possibility, Pelosi said Thursday that opponents of a public option should be careful of what they wish for.

"If they want no public option but a trigger, you can be sure that the trigger will bring on a very robust public option," Pelosi said. "So if I were advising the insurance companies, I would tell them take this bill as it is now, because if you don't perform and ... there is a trigger, it's not going to be one of those little flags that come out of the end of the barrel."

President Obama will address a joint session of Congress on Wednesday night to talk about health care, and his chief spokesman said Sunday the speech would spell out the president's ideas for a bill. However, White House spokesman Robert Gibbs provided no details of what Obama will say.

"People will leave that speech knowing where he stands," Gibbs said on the ABC program "This Week."

Asked whether Obama, a strong proponent of a public option, would veto a final bill that lacks the provision, Gibbs avoided a direct answer.

"We're not going to prejudge what the process will be when we sign it, as the president expects to do this year," he said, later adding: "I doubt we're going to get into veto threats" in Wednesday's speech.

On the same program, a Democratic congresswoman said Republicans have no intention of supporting a health-care bill and only want to use the issue to damage Obama politically.

Rep. Maxine Waters of California said Democrats must push for the bill they want because Republicans don't want any bill to pass.

"You don't have a bill. Where's your bill? What have you come up with? What are you offering as an alternative?" Waters asked Republican Rep. Mike Pence of Indiana, another guest on the show. "It's bigger than health care. It's about President Obama, and the Republicans have decided to use this by which to bring him down."

Pence denied Water's accusation, saying Republicans seek a less comprehensive and more affordable bill than the Democratic proposals in the House and Senate.

Americans are concerned about Obama's policies to expand government and increase spending, Pence said, adding that the Democratic health-care proposals will bring a government takeover of health care.

Asked to explain how, he said companies that provide health-care coverage as a benefit for workers would choose drop that coverage and pay a fine, shifting most people to the public insurance option.

"In this economy, there's no small business or large business worth its salt that isn't going to take a hard look at sending all of their employees to the federal government for their health insurance the minute a public option is available," Pence said.

In response, former House Majority Leader Tom Daschle, a Democrat also appearing on the show, denied that a public option would have a competitive advantage to drive private insurers out of the market.

In view of such vitriol and what it called Republican intransigence, The New York Times said in an editorial Saturday that Obama and congressional Democrats should use a legislative tactic called "budget reconciliation" to pass a health-care bill with a smaller majority than needed for other bills.

Republicans warn that approving the bills through reconciliation -- which requires 51 Senate votes rather than the 60 needed to avoid a filibuster -- would be a declaration of political war.

Democratic legislators said Sunday they would prefer a bill with bipartisan support that could pass without reconciliation, but they refused to rule out the tactic.